The preschool years are an important time for implementing strategies to prevent obesity. Preschool children are thought to be in a formative stage during which the ability to regulate energy (calorie) intake is being replaced by eating in response to environmental cues such as the properties of foods. In controlled studies, we have found that increases in food portion size (weight) and energy density (calories per weight) promote excess energy intake in preschool children across several meals. The proposed experiments will extend this work by systematically varying the foods and beverages served over multiple days to determine whether children's regulation of energy intake counters the effects of large portions of energy-dense foods or whether such foods have persistent effects on energy intake. We also will test a strategy for moderating energy intake based on current dietary guidance, which uses portion size in a positive way to increase the proportion of low energy-dense foods such as vegetables and fruits and to decrease the proportion of high energy-dense foods. In this proposal we will conduct three studies that vary the properties of foods served over 5 consecutive days to children 3 to 5 years of age in their preschool and home environments. Experiment 1 will compare serving larger portions of all foods and beverages to serving smaller portions of all items, and Experiment 2 will compare serving higher-energy-dense versions of all foods and beverages to serving lower-energy-dense versions. Experiment 3 will compare serving meals with typical proportions of food groups to serving meals in which larger portions of low-energy-dense vegetables and fruits are substituted for foods that are higher in energy density. Thus, the three specific aims are to determine the effects on energy intake of varying the portion size, the energy density, and the proportion of lower-energy-dense foods of all meals served over 5 days. The overall goals of the research are to evaluate (a) whether preschool children adjust their energy intake over multiple days to compensate for variations in food portion size and energy density, thus showing self-regulation, and (b) whether these powerful food properties can be used positively to manage energy intake in the current obesogenic environment. Well-controlled studies such as these will lead to the development of evidence-based strategies to help moderate energy intake and prevent childhood obesity.